Time of Flight Mass Spectrometry (TOF-MS) is rapidly becoming the most popular method of mass separation in analytical chemistry. This technique is easily deployed, can produce very high mass resolution, and can be adapted for use with many forms of sample introduction and ionization. Unlike quadrupoles and ion traps, time of flight mass analyzers perform well at very high mass. Descriptions of described time of flight analyzers maybe found in Wiley and McLaren (Rec. Sci. Instrum., 26, 1150 (1950)), Cotter (Anal. Chem., 1027A (1992)), and Wollnik (Mass Spectrom Rev., 12, 89 (1993)).
Time of flight mass spectrometers are produced in two main configurations: linear instruments and reflectron instruments. In operation of either configuration of mass spectrometer an unknown sample is converted to ions. For example, a sample may be ionized using a MALDI (Matrix Assisted Laser Desorption Ionization) instrument 100, as illustrated in FIG. 1. The ions created by laser ionization of the sample are injected into a flight tube 10 where they begin traveling towards a detector 20. The motion of the ions within the flight tube 10 can be described by:t2=m/z (d2/2Vse),  (1)
where m/z is the mass to charge ratio of the ion, d is the distance to the detector 20, and Vse is the acceleration potential. The lighter ions (low mass) travel faster than the higher mass ions and therefor arrive at the detector 20 earlier than the higher mass ions. If the flight tube 10 is long enough, the arrival times of all of the ions at the detector will be distributed according to mass with the lowest mass ions arriving first, as shown in FIG. 2.
When the ions arrive at the detector 20, e.g., a multi-channel plate detector, the ions initiate a cascade of secondary electrons, which results in the generation of very fast voltage pulses that are correlated to the arrival of the ions. A high-speed oscilloscope or transient recorder maybe used to record the arrival times. Knowing the exact arrival times, equation (1) can be used to solve for the mass to charge ratio, m/z, of the ions.
The second type of time of flight mass spectrometer is a reflectron instrument 300 as shown in FIG. 3. The reflectron design takes advantage of the fact that the farther the ions are allowed to travel, the greater the space between ions of differing masses becomes. Greater distances between ions with different masses increase the arrival time differences between the ions and thereby increase the resolution with which ions of a similar m/z can be differentiated. In addition, a reflectron design corrects the energy dispersion of the ions leaving the source.
The reflectron instrument 300 includes a reflectron analyzer 350 comprising a flight tube 310, reflectron lens 330, and a detector 320. The flight tube 310 includes a first, input end 315 at which the detector 320 is located and a second, reflectron end 317 at which the reflectron lens 330 is located. The ions are injected into the flight tube 310 at the input end 315 in a similar manner as a linear instrument. However, rather than detecting the ions at the opposing second end 317 of the flight tube 310, the ions are reflected back to the input end 315 of the flight tube 310 by the reflectron lens 330 where the ions are detected. As shown in FIG. 3, the ions travel along a path “P” which effectively doubles the length of the flight tube 310.
The reflection of the ions is effected by the action of an electric field gradient created by the reflectron lens 330 along the lens axis. Ions traveling down the flight tube 310 enter the reflectron lens 330 at a first end 340 of the reflectron lens 330. The electrostatic field created by applying separate high voltage potentials to each of a series of metal rings 332 of the lens 330, slows the forward progress of the ions and eventually reverses the direction of the ions to travel back towards the first end 340 of the lens 330. The ions then exit the lens 330 and are directed to the detector 320 at the first end 315 of the flight tube 310. The precision ground metal rings 332 are stacked in layers with insulating spacers 334 in between the metal ring layers. The rings 332 and spacers 334 are held together with threaded rods. This assembly may have hundreds of components which must be carefully assembled (typically by hand) in a clean, dust free environment. Such a lens assembly having many discrete components can be costly and complicated to fabricate. Moreover, the use of discrete metal rings 332 necessitates the use of a voltage divider at each layer of rings 332 in order to produce the electrostatic field gradient necessary to reverse the direction of the ions.
Accordingly, it would be an advance in the state of the art to provide a reflectron lens having a continuous conductive surface and which could introduce an electric field gradient without the use of multiple voltage dividers.